Low temperature refrigeration

ABSTRACT

860,723. Liquefaction of gases. CONCH INTERNATIONAL METHANE Ltd. Aug. 31, 1959, No. 29696/59. Class 8(2). Liquefaction of a first gas e.g. methane by heat exchange with a colder second gas of lower boiling point, e.g. nitrogen is effected by passing compressed methane through passages 22, 16, 10 of series arranged heat exchangers 6, 4, 2 in countercurrent to pressurized liquid nitrogen forced by a pump 28 through passages 8, 14, 20 and also in countercurrent to nitrogen gas exhausted from the passage 20 of exchanger 6 after expansion and consequent cooling in a turbine 36 and traversing passages 12, 18 of exchangers 4, 6. As shown the mass flow rates of the methane and nitrogen are equal and the extra cold requirement is met by an external refrigerator 26 driven by the turbine 36 and exhausting through a passage 24 in exchanger 6.

Oct, 27, 1959 P. E. BOCQUET ETAL LOW TEMPERATURE REFRIGERATION Filed Aug. 29, 1955 A TTORNE Y of liquefying United States Patent O 2,909,906 LOW TEMPERATURE REFRIGERATION Philip E. Bocquet and Shao E. Tung, Ponca City, Okla., assignors, by mesne assignments, to Constock Liquid Methane Corporation, New York, N. a corporation of Delaware l Application August 29, 1955, Serial No. 531,125

11 Claims. (Cl'. 62-88) This invention relates to improvements in the art of refrigeration, and more particularly, but not by way of limitation, to an improved refrigeration process of liquefying methane by use of initially liquid nitrogen.

The present invention has conveniently definable utility when applied to the liquefaction of methane by nitrogen. Therefore, the invention will be described with detailed reference to these particular fluids. It is to be understood, however, that the present process is not limited to the use of these fluids, but may be practiced with various other uids having similar relations, as will be more fully hereinafter set forth.

Methane frequently exists in a gaseous state at approximatelyatmospherie pressure and 90 F. when it is desired to liquefy the methane for convenience in storage and/or transportation. Approximately 398 B.t.u. of refrigeration are required to liquefy each pound of methane from these conditions. Liquid nitrogen may be conveniently used to obtain at least a portion of this refrigeration. The liquid nitrogen may be obtained at approximately atmospheric pressure and at 320 F., and will absorb approximately 168 B.t.u./lb. in being heated to 80 F. Following present teachings, the liquefaction would be accomplished by simply heat interchanging the nitrogen and methane at atmospheric pressure. The methane would be liquefied by such a process, but either approximately 2.1 pounds of nitrogen must be used for liquefactionof each pound of methane, or a substantial amount of external refrigeration must be used, since the methane will require 398 B.t.u./lb. refrigeration and the nitrogen supplies only 186 B.t.u./ lb.

The present invention contemplates a process of liquefying methane by nitrogen, wherein one pound of methane may be liquefied by each pound of nitrogen with only a minor use of external refrigeration; and wherein the process produces mechanical power which may be used in the production of the external refrigeration. It is proposed to pressurize the liquid nitrogen, and then provide two heat exchanges of the nitrogen with a stream of methane. It is proposed to feed the methane at a pressure as near to well head pressure as possible. lf the pressure is too low, the methane may be compressed prior to the refrigeration thereof, to obtain a better balance inv the demand and supply of refrigeration during the process.

An important object of this invention is to increase the refrigeration which may be obtained by an initially liquid refrigerant.

Another object of this invention is to provide a process a gas stream by refrigeration, wherein the mass rates of the stream and the refrigerant are substantially equal.

Another object of this invention is to provide a process of liquefying a gas stream by refrigeration, wherein the refrigerant produces mechanical power during the process.

A further object of this invention is to provide an economical process of liquefying a gas stream which may b simply performed by standard equipment.

2,999,996 Patented Oct. 27, 1959.

Another object of this invention is to provide a process of liquefying a gas stream by an initially liquid refrigerant, wherein the refrigerant is interchanged with the stream in two passes of the refrigerant 'through substantially equal temperature zones.

A still further object of this invention is to provide a process of liquefying a gas stream by interchange with a refrigerant and a supplemental refrigerant, wherein the mass rates of the stream and refrigerant are equal, yet the refrigerant supplies the major portion of the required refrigeration.

Still another object of this invention is to provide a process of liquefying a gas 'stream by interchange with an initially liquid refrigerant, wherein the refrigerant is cooled at an intermediate point in the process for a second interchange with the stream.

Other objects and advantages of the invention will be evident from the following detailed description, when read in conjunction with the accompanying drawing, which illustrates our invention.

In the drawing, the single figure is a ilow diagram illustrating one application of the present invention.

Broadly stated, the present invention may be defined as the method of refrigerating a rst fluid which comprises:

(a) Moving a stream of said iirst fluid in counterllow heat exchange relation with (b) a pressurized initially liquid stream of a second fluid whose temperature at any point in the system is below the temperature of said rst iluid at the same point,

(c) maintaining substantially constant the pressure of said second stream until its temperature has reached a level at which, when the stream has been expanded to a predetermined lower pressure, it will exist essentially in the gaseous phase, (d) causing said second stream to expand to said predetermined pressure in a work-producing zone wherein its temperature is substantially reduced, and then (e) moving said thus cooled second stream in counterflow heat exchange relation with said rst stream.

Referring to the drawings in detail, reference characters 2, 4, and 6 designate three heat exchangers arranged in side-by-side relation. The smaller heat exchanger 2 has two flow passageways 8 and 10 therethrough for the passageof the refrigerant and the stream being liquefied, respectively, as will be more fully hereinafter set forth. The central heat exchanger 4 has three llow passageways 12, 14, and 16 therethrough, and the larger heat exchanger 6 has four ilow passageways 18, 2i), 22, and 24 therethrough. lt will be understood that the heat exchangers 2, 4, and 6 are shown diagrammatically and that the various liow passageways may be arranged in any suitable manner to obtain the desired refrigeration, as will be subsequently described.

The apparatus required for practicing the present invention also includes a source of outside refrigeration 26, which communicates with the flow passageway 24 of the heat exchanger 6 to provide supplemental refrigeration of the methane stream during the initial stages of the vre` frigeration. A compressor 2S communicates with the llow passageway 8 of the smaller heat exchanger 2 and may be of any suitable type which can pressurize an initially liquid stream to a substantial pressure.

The gas stream to be liqueed (which will be described as methane) is conducted sequentially through the refrigeration flow passageways 22, 16, and 10 by suitable piping 30. The refrigerant (which will be described as nitrogen) is conducted through the compressor 28 and llow passageways S, 14, and 20 by suitable piping 32. Also, the outlet of the flow passageway 20 of the largest heat exchanger 6 communicates with the inlet -of the flow passageway 12 of the central heat exchanger 4 through suitable piping 34. A mechanical expansion device 36 is interposed in they conduit 34 to expand the refrigerant, as will be more fully hereinafter setforth. The mechanical expansion device 36 is preferably a turboexpander in orderrthat useful power may be generated by the expansion of the refrigerant.

The heat contents of methane and nitrogen at various points in our process as shown in the drawing and set forth in'this description were determined from the following references:

V(a) For methane-Transactions of The American Institute of Chemical Engineers, 42, 55 (1946). I V(b) For nitrogen-US. Bureau of Mines Technical Paper 424.

sv previously stated, methane frequently exists in a gaseous state at approximately atmospheric pressure and A90 F. prior to liquefaction thereof. Also, liquid nitrogen may be obtained at atmospheric pressure and approximately 320 F. Prior to the conduction of the methane into the larger heat exchanger 6, the methane is preferably compressed to a pressure above its critical pressure, such as 1,000 p.s.i.a., as indicated in the drawing. The methane will then exist in a uid state at approximately 90 F. and will have a heat content of approximately 388 B.t.u./lb.

In accordance with the present invention, the methane is conducted sequentially through the heat exchangers 6, 4, and 2 (in that order) to lower the temperature of the methane and reduce the heat content thereof, whereby the methane will be discharged from the flow passageway 10 of the heat exchanger 2 in a liquid state. The pressure of the methane may be retained at 1,000 p.s.i.a. throughout its ow through the various heat exchangers and then expanded through a suitable throttling valve (not shown) for reduction to approximately atmospheric pressure. In that event, it will beunderstood that the methane discharging from the smaller heat exchanger 2 will exist in a uid state above its critical pressure, rather than in a liquid state. However, since the heat content of the methane at 1,000 p.s.i.a. (as it discharges from the heat exchanger 2) is substantially equal to the heat content of liquid methane at atmospheric pressure, the methane may be throttled to atmospheric pressure without any appreciable gas and refrigeration being produced. It will also be understood by those skilled in the art that Athe pressure of the methane may be reduced in steps between the successive heat exchangers to obtain a liquid methane discharging from the flow passageway 10 of the last heat exchanger 2, as long as such pressure drops do not upset the supply and demand balance between the exchanging streams.

The liquid nitrogen is pressurized by the compressor 28 to an elevated pressure, preferably above its critical pressure, such as 90 atmospheres. The nitrogen will then have a temperature of approximately 315 F. and a heat content of approximately 10 B.t.u./1b. It will be noted that this pressurization takes Vplace prior to any heat exchange by the nitrogen.

kThepressurized nitrogen is then passed sequentially through the heat exchangers 2, 4, and'6 via the ow passageways 8, 14, and 20, respectively, Ato obtain a transfer of heat from the methane to the nitrogen in each of the heat exchangers. It will be observed from the various process 'conditions on the drawing that a temperature difference exists between the nitrogen and methane in each of the heat exchangers to'provide a transfer of heat from the methane to the nitrogen. When the nitrogen is discharged from the last heat exchanger 6, it will exist at approximately 80 F. and at a pressure approximating 90 atmospheres. Thus, the first passage of the nitrogen through the heat exchangers is made at substantially a constant elevated pressure.

The heated nitrogen is transferred through the conduit 34 tothe inlet of the turboexpander 36 at its elevated pressure. As the nitrogen is expanded in the turboexpander 36, the pressure of the nitrogen is reduced to approximately atmospheric, and the temperature of the nitrogen will be reduced to approximately 233 F. The expansion of the nitrogen provides approximately 70 B.t.u./lb. of power which will be developed by the turboexpander and utilized as will be subsequently described. The cooled nitrogen is then passed through the heat exchangers 4 and 6 via the flow passageways 12 and 18 to provide a further cooling of the methane. The temperature of the cooled gaseous nitrogen in the ow passageways 12 and 18 will be substantially the same as the temperature of the compressed nitrogen passing through the passageways 14 and 20, respectively, toprevent a heat transfer between the passageways 12 and 14, and the passageways 18 and 20, respectively. The nitrogen gas exhausting from the flow passageway 18 of the larger heat exchanger 6 vmay be utilized in any desired manner.

The various process calculations indicated on-the drawings are made on the basis that the mass rates of the methane and nitrogen are equal. That is, a pound of methane is liquefied for each pound of nitrogen passed through the heat exchangers 2, 4, and 6. It will be observed that the nitrogen absorbs a total of 245 B.t.u./1b. within the heat exchangers 2, 4, and 6; whereas, the methane must give up a total of 370 B.t.u./lb. for liquefaction. Therefore, B.t.u./lb. (of methane) Vadditional refrigeration must be utilized. .The' additional refrigeration is obtained by the outside refrigeration vsource 26 through the medium of the How passageway 24 in the largest heat exchanger 6. The kind of refrigerant circulated through the tlow passageway 24 is immaterial, except that it must be colder than the methane to provide a transfer of heat from the flow passageway 22 ito the ow passageway 24. Also, the temperature of the additional refrigerant owing through the flow, passageway 24 should be equal to the temperature of the nitrogen passing through the passageways 18 and 20'to prevent a transfer of heat between the two types of refrigerants. It will be observed, however, that the vadditional refrigeration is obtained at a relative high temperature level, therefore the more common refrigerants may be used.

The 70 B.t.u./1b. of power obtainable from the turboexpander 36 may be conveniently used in the operation of the outside refrigeration source 26. Also, a portion of the power obtainable from the turboexpander 36 is preferably used in driving the nitrogen compressor 28. However, the power required for driving the compressor Y28 will be relatively small since the nitrogen is in liquid form at this pointin the process and very little work is required to compress or pressurize a liquid. Therefore, the major portion of the power obtainable from the .turboexpander 36 may be utilized withV the source of outside refrigeration 26, thereby requiringl limited additional power for' supplying the supplemental refrigeration through the How passageway 24. Y

The present process is primarily designed for a system wherein liquid methaneis produced .at one point and liquid nitrogen is producedat 'a distant point and it is desired tointerchange theY methane iwith `the nitrogen. The liquid methane and liquid nitrogenare transported in the Vsame ships, and to maintain an economical transportation system, the liquid methane and nitrogen should be 'transported in equal weight amounts. Therefore, the present process has -been described using equalma'ss rates of methane and nitrogen. The methane can be liquefied by the use of nitrogenronly,` that is, without the use of outside refrigeration. However, approximately an additional one-half pound ofV nitrogen is required to liquefy each pound of methane. It is more economical `to use equal mass rates and outside refrigeration, inasmuch as the tonnage requirements of the transporting ships would need be increased, even-if excess liquid nitrogen is available at the distant point. This last statement is true becau$a the powerobtainablefrom the expansion turbine 36 will be substantially sucient to supply the power necessary for the pump 28 and the refrigeration unit 26. Such additional refrigeration as may need to be powered from a truly outside source can be secured economically because of the temperature level at which such outside refrigeration is required.

As stated at the outset of this specification, various fluids may be used in the process of this invention. Therefore, the invention is not limited to the use of nitrogen as the refrigerant and methane as the stream being liquefled. Other fluids may be used, providing the fiuid being used as a refrigerant is colder than the stream being liquefied when both are in liquid form. Typical examples of substitute fiuids which may be used in lthe present process are set forth in the following tabulation:

Refrigerant: Fluid being liquefied Hydrogen Nitrogen Air Methane Methane Ethylene Methane Propane Methane Butane Nitrogen Oxygen From the foregoing, it is apparent that the present invention increases the refrigeration which may be obtained by an initially liquid refrigerant. By pressurizing the liquid refrigerant, the refrigerant may be passed in heat exchange relation with the stream being liquefied to obtain one series of heat transfers from the stream to the refrigerant. The warmed refrigerant may then be expanded and cooled through a work-producing zone; whereupon the refrigerant may be passed in a second heat exchange with the stream to obtain an additional refrigeration of the stream. The process is economical and may be simply performed by standard equipment. The process is not limited to the use only when the mass rates of the stream and refrigerant are equal; however, when they are equal only a minor amount of outside power is required for supplemental refrigeration in order to liquefy the stream. In addition, the present process provides mechanical power which may be used in producing the supplemental refrigeration.

While particular embodiments of the invention have been described, it will be understood, of course, that the invention is not limited thereto since may modifications may be made, and it is, therefore, contemplated to cover by the appended claims any such modifications as fall within the true spirit and scope of the invention.

The invention having thus been described, what is claimed and desired to be secured by Letters Patent is:

l. The method of refrigerating a first fluid which comprises: (a) pressurizing a second liquid to above its critical pressure for presentation as a fiuid whose temperature at any point in the system is below the temperature of said first fiuid at the of said first fiuid in counterfiow heat exchange relation with said second iiuid stream, (c) maintaining substantially constant the pressure of said second fluid stream until its temperature has reached a level at which, when the stream has been expanded to a predetermined lower pressure with resulting drop in temperature, it will exist essentially in the gaseous phase at the lower temperature, (d) causing said second stream to expand to said predetermined pressure in a work-producing zone wherein its temperature is substantially reduced, (e) moving said thus cooled second stream now in the gaseous phase in counterow heat exchange relation with said rst fluid stream utilizing the work produced in said work producing zone to pressurize said second stream.

2. The method of refrigerating the first fluid which comprises: (a) moving a stream of said first fiuid in counterfiow heat exchange relation with (b) a pressurized initially liquid stream of a second fluid whose temperature at any point in the system is below the temperature of said first fiuid at the same point, (c) maintaining subsame point, (b) moving a stream stantially constant the pressure of said second fluid stream until its temperature has reached a level at which, when the stream has been'expanded to a predetermined lower pressure with resulting drop in temperature, it will exist essentially in the gaseous phase at the lower temperature (d) causing said pressurized second stream of fluid to expand to said predetermined pressure in a work producing zone wherein its temperature is substantially reduced, (e) moving said thus cooled second stream in counterflow heat exchange relation with said first stream, and then (f) using the energy derived in said work producing zone to additionally refrigerate said first stream.

3. In the method for the refrigeration of the first gas by passage in heat exchange relation with a low boiling second gas in at least two separate passes in one of which the second gas remains in a fluid state and in the other of which the second gas is in a gaseous state comprising the steps of supplying said second gas in a uid state at high pressure and low temperature, passing said first gas in heat exchange relation with said iiuid stream of the second gas for the removal of cold to refrigerate the first gas andto raise the temperature of the iiuid stream of second gas by an amount to cause said fiuid stream of the second gas to be converted to a gaseous state upon subsequent expansion to a lower pressure with resulting reduction in temperature to make a `large amount of cold gas available for refrigeration, expanding said fluid stream' of said second gas to a lower pressure for` conversion of the iiuid stream of the second gas to a gaseous state with resulting reduction in temperature to provide a largeamount of cold gas for refrigeration, passing said first gas in heat exchange relation with the expanded stream of said second gas for the extraction of heat from said first gasto refrigerate the first gas.

4. In the method for the refrigeration of a first gas by passage in heat exchange relation with a lower vboiling second gas in two separate streams in one of which said second gas remains in a fluid state and in the other of which said second gas is in a gaseous state comprising the steps of compressing said second gas to a pressure above its critical pressure whereby it remainsl in a fiuid state at high pressure and low temperature', passing the fluid stream of said second gas in heat exchange relation with said first gas to lower the temperature of said first gas while raising the temperature of the iiuid stream of second gasto a level where said fiuid stream of second gas will be converted to a gaseous state upon expansion to lower pressure, expanding said fluid stream of said second gas to a lower pressure with work whereby said stream of said second gas is converted to a gaseous state with resulting reduction in temperature to make a large amount of cold gas available for refrigeration, and passing said expanded stream of said second gas in a gaseous state in heat exchange relation with said stream of first gas whereby cold is extracted by said stream of first gas from said stream of second gas for further reduction in temperature.

5. The method as claimed in claim 4 in which each pass of the first stream of gas is passed in heat exchange relation with the streams of the second gas in a series of heat exchange steps for stepwise increase in the temperature of the stream of said second gas and stepwise decrease in temperature lof said stream of said first gas.

6. The method as claimed in claim 4 in which the fluid stream of said second gas is expanded to atmospheric pressure for maximizing reduction in temperature.

7. The method as claimed in claimed 4 in which the stream of said first gas is compressed to a pressure above its critical pressure prior to passage in heat exchange relation with the separate streams of said second gas.

8. The method as claimed in claim 4 which includes the step of passing the stream of first gas in heat ex- 7 change relation with a separate refrigerant for the additional removal of heat.

9. In the method of 'refrigerating natural gas by pas- 'sage .ih vheat exchange relation with a lower boiling gas in at least two separate passes in one of which the second 'gas is in a fluid state and in the other of which said seco'nd gas is in a gaseous state, comprising the steps of compressing said second gas to a pressure above the critical pressure of said second gas whereby the latter exists in a fluid state at low temperature, passing the iid stream `of said second gas in heat exchange relation with the natural gas to lower the temperature of thenatu'ral gas while raising the temperature of said iluid stream of second gas to a level where said uid 'stream will be converted to a gaseous state upon expasion to lower pressure, expanding said fluid stream of said second gas to a lower pressure with work whereby said stream of second gas is converted to a gaseous state with resulting reduction in temperature to make a large amount of cold gas available for refrigeration, and passing said c'old and expanded stream of the second gas in heat exchange relation with the refrigerated stream of the natural gas whereby cold is further extacted by the stream of the iirst gas from the second for further reduction in temperature.

10. In the method of liquefaction of a natural gas by the passage in heat exchange relation with a lower boiling second gas in at least two separate streams in one of which said lower boiling gas is in a fluid state and in the other of which said lower boiling gas is in a gaseous state comprising the steps of compressing said lowe'r boiling gas to a pressure above its critical pressire whereby it exists in a fluid state at low temperature, passing the fluid 'stream of said lower boiling gas in heat exchange relation with the stream of natural gs to lower the temperature of the natural gas while raising the temperature of the uid stream of the lower boiling gas to a level where the latter will be converted to 'a` gaseous state upon expansion to lower pressure, expanding said uid stream of the lower boiling gas 'to af lower pressure with work whereby the tluid stream of the lower boiling gas is converted to a gaseous state with resulting drop in temperature to make a large amount of vcold gas available for refrigeration, and passing the cold expanded streamof the lower boiling gas in heat exchange relation with the stream of natural gas whereby the latter extracts cold from the expanded lower boiling gas for further reduction in temperature.

ll. In the method of liquefaction of a natural gas composed mostly of methane by passage in heat exchange relation with a stream of a lower boiling gas selected from the group consisting of air and nitrogen in at least two separate streams, in one of which the lower boiling gas is present in a uid state at high pressure and in the other of which the lower boiling gas is present in a gaseous state comprising the steps of compressing said lower boiling gas to a pressure above its critical temperature to provide the gas in a fluid stream at low temperature, passing the lluid stream of the lower boiling gas in heat exchange relation with the natural gas to lower the temperature of the natural gas while simultaneouslyraising the temperature of the uid stream to a level where the fluid stream will be converted to the gaseous state upon subsequent expansion to lower pressure, expanding the iluid stream with work to a lower pressure whereby said stream is converted from a uid state to a gaseous state with resulting reduction in temperature to provide a large amount of cold gas for refrigeration, and passing the cold and expanded stream of the lower boiling gas in heat exchange relation with 'the stream of natural gas whereby cold is extracted by the natural gas stream from the cold and expanded lower boiling gas for further reduction in temperature.

References Cited in the tile of this patent UNITED STATES PATENTS 655,148 Dickerson July 31, 1900 683,010 Bobrick Sept. 17, 1901 2,458,894 Collins Jan. l1, 1949 2,685,180 Schlitt Aug. 3, 1954 2,685,181 Schlitt Aug. 3, 1954 FOREIGN PATENTS 736,736 France Sept. 26, 1932 

